Field of the Invention
The present invention concerns a method for acquiring magnetic resonance image data of an object under investigation by the use of a magnetic resonance sequence that includes at least one adiabatic radio-frequency pulse, as well as a magnetic resonance apparatus and a non-transitory, computer-readable data storage medium encoded with programming instructions to implement such a method.
Description of the Prior Art
In a magnetic resonance apparatus, also called a magnetic resonance tomography system, usually the body of a subject undergoing investigation, in particular a patient, is exposed to a relatively strong basic magnetic field, for example of 1.5 or 3 or 7 tesla, by operation of a basic field magnet. In addition, a gradient coil arrangement is used to apply gradient fields. Suitable radio-frequency antennas are used to emit radio-frequency pulses, for example excitation pulses, and this results in nuclear spins of certain atoms that are excited to resonance by these radio-frequency pulses being tipped by a defined flip angle in relation to the magnetic field lines of the basic magnetic field. During relaxation of the nuclear spins, radio-frequency signals—so-called magnetic resonance signals—are emitted and these are received using suitable radio-frequency antennas, and then subjected to further processing. Finally, the desired image data can be reconstructed from the raw data that is acquired in this way.
Thus, for a particular measurement, it is necessary to emit a magnetic resonance sequence, also called a pulse sequence that includes a sequence of radio-frequency pulses, for example excitation pulses and refocusing pulses, and gradients that are switched in a manner coordinated therewith, on different gradient axes in different spatial directions. Timed to coincide with this, readout windows are defined to predetermine the periods during which the induced magnetic resonance signals are acquired.
In this context, a magnetic resonance sequence may provide for the use of at least one adiabatic radio-frequency pulse. Adiabatic radio-frequency pulses are known for example from the paper by Tannus et al., “Adiabatic Pulses”, NMR in Biomed, 10, 423-434 (1997). In general, an adiabatic radio-frequency pulse is a pulse whose frequency or phase is modulated. At the same time, the adiabatic radio-frequency pulse may include modulation of an amplitude. An adiabatic pulse may be designed to excite all the nuclear spins of an investigated area of an examination object to the same extent. In this case, an adiabatic radio-frequency pulse typically has the advantage that excitation of the nuclear spins and/or a flip angle of the adiabatic radio-frequency pulse is insensitive to a certain insensitivity range of B0 variations and/or B1 variations. Because of this, an adiabatic radio-frequency pulse may be suitably used even when there are inhomogeneities in the magnetic field.